1. Technical Field
The invention relates to devices and methods for depleting dissolved gases and for separating the gas phase from gas-water phase mixtures from waterways as well as the use of these devices and methods.
2. State-of-the-Art
The last 200 years of global industrial development have led to a drastic exploitation of fossil energy sources generated during geologic history. A large number of the worldwide deposits of crude oil, gas and coal will be depleted in the foreseeable future. Nevertheless, the energy demand of the world population has an undiminished progressive trend (Hawksworth, J.: The World in 2050. Can rapid global growth be reconciled with moving to a low carbon economy?, PricewaterhouseCoopers LLP—July 2008. 1-21).
The exploitation of methane hydrate deposits discovered at the shelf edges, i.e., at a large depth, is currently not yet practiced due to different aspects, such as for example the tremendous demands on the technology or the high recovery-related risk of spontaneous outgassing effects with global relevance for the climate (Zhang, Y., Kling, G. W.: Dynamics of Lake Eruptions and Possible Ocean Eruptions. Annu. Rev. Earth Planet. Sci. 2006.34: 293-324).
In addition to technologies which are so far not in existence and would allow the use of gaseous raw materials residing in waterways, such “deposits” can also represent risks.
The climatic development of the earth confirms global warming independent of the discussion of the underlying causes. Recent geological discoveries support a new theory which explains unchallenged the short-term global extinction of the species on earth. In contrast to meteor impacts, volcanic eruptions, etc., this theory bases the extinction on the hypothesis of massive H2S-outgassing of the oceans (Berner, R. A.: Plants, H2S, CO2, O2 and the Permo-Triassic Extinction. 2006 Philadelphia Annual Meeting (22-25 Oct. 2006) Philadelphia, Pa., Paper No. 137-9). The mechanisms of the associated phase separation are known. The key mechanism is generally coupled to global warming. The triggers for such eruptive phase separations can be of different in nature, for example volcanic eruptions, phase-separation-related (methane) gas eruptions, anthropogenic greenhouse effect, etc. The illustrated mechanism has already been observed in smaller waterways (example of already significant size: latest fish extinction in the Baltic Sea). The phase-separation-driven lake eruptions are known from different lakes in Africa and have caused terrible catastrophes (Zhang, Y., Kling, G. W.: Dynamics of Lake Eruptions and Possible Ocean Eruptions. Annu. Rev. Earth Planet. Sci. 2006. 34: 293-324).
The controlled reduction in the concentration of climate-relevant gases in deep waterways would reduce the risk of ocean gas eruptions and the introduction of climate-relevant gases into the atmosphere. At the same time, the available gas storage capacity of the oceans would also tend to be increased.
An efficient process could allow such a reduction depending on the quantity of gas present and its composition as a cost-effective remediation or also as a profitable exploration of the gas deposit “Ocean.”
One problem with the recovery of gases from waterways is that, driven by the pressure and phase density differences, a continuous gas phase separation and the enrichment of the gas-water phase mixture are highly nonlinear processes which can occur in a self-reinforcing manner (in open waters also explosively) and therefore place high demands on material and extraction technology and are associated with high accident risks. Zhang & Kling describe the free buoyancy-related movement of a developing gas phase in waterways and its movement in a pipe as a process with positive feedback. The beginning of the degassing causes a decrease in the density of the mixture, and hence buoyancy. Due to the rise of the gas-water mixture, regions with lower ambient pressure are reached, causing the pressure in the gas-water mixture to decrease further, thus releasing additional gas. A methane gas-water mixture may cause an explosive rise in spite of the low solubility of methane in water. Higher solubilities, for example for CO2 or H2S, result in a significantly stronger feedback. Zhang & Kling estimate the velocity at which to center of a methane bubble cloud (1%Mass CH4) reaches the water surface from 500 m depth at 130 m/s. The velocity maximum still reaches 62 m/s for 0.1%Mass. In this case, 0.1%Mass CH4 under standard conditions corresponds to approximately 22.4 L/mole*1 g (CH4)/16 g/mole=1.4 L. Such gas-water phase mixture is at the water surface volumetrically composed of approximately one half gas phase (phase fraction of 0.58). The mechanical energy density 1/2 ρν2=ρgh results in a height of the gas mixture in the order of h=ν2/(2 g)≈102 m and thus causes an explosive discharge at the water surface.
It is an object of the present invention to alleviate or solve one or several of the aforementioned problems.